Annealing of cold-worked aluminium (2nd report)

Abstract

Annealing of cold-worked metals at proper temperatures produces recrystallization followed by recovery and growth of these recrystallized grains. They have the following characteristics:Recrystallization-(1) Nucleus boundaries appear around the places where recovery has been Perfect in a large block. (2) The heavier the working, the easier the recrystallization occurs. The size of nucleus becomes smaller, the number of it increases, and the size ratio of nucleus to sub-grain decreases with an increase in the rate of working. (3) The nucleus size in the lightly worked state is approximately equal to the sub-grain size, and that in the heavily worked state can grow up rapidly to the sub-boundary by the action of numerous polygonization walls. (4) At the beginning of recrystallization, the recrystallization boundaries lack in clearness. (5) The grain size of recrystallization is a unique function of the rate of working. (6) The higher the annealing temperature, the lower the value of micro-hardness immediately after the recrystallization. (7) There are no recrystallization nucleus in the metals cold-tensioned more lightly than 20%, but they are recovered by stress-induced migration of already existing boundaries. (8) At the beginning of recrystallization there is found a large local difference in micro-hardness and in strain-relief at etching pits in the heavily worked metals as well as in the lightly worked ones. (9) The banded structure is not subject to any prominent change from the recovered state, and it remains until the next coarsening stage.Growth of recrystallized grain, which was conventionally called "secondary recrystallization" or "coarsening", is customarily considered to occur after a certain incubation period. But, actually, the growth proceeds already in this period. Each recrystallization grain is an ordinary unit in this growth which depend mainly on the capillary tension of polygonization walls in respective grains, and each process is finished in a relatively short time.In this case, the growth of grain having a crystal plane near (100) on the specimen surface is prominent along the rolling direction, and that of grain having a crystal plane near (111) is prominent in the perpendicular direction. The frequency of a crystal plane near (111) on the specimen surface increases a little owing to this process.Thus, the duty of polygonization walls is important at the initial stage of this growth, but the analogous effect as seen in the ordinary grain growth become gradually prominent. When the grown-grain size reaches 10 or more times of average recrystallized grain size, the rate of growth increases rapidly by the effect of ordinary grain growth. This is the coarsening which takes place at first preferentially along the rolling direction.The original boundary in growth process mentioned above needs some time for its perfect disapperance after growth.To the above two stages of growth the auther, for convenience sake, has given names the "short range annexation" and "long range annexation" according to their mechanisms respectively.Lightly worked metals show quite different appearances because they have no short range annexation and have relatively large nuclei. Metals cold-tensined smaller than 20% go directly to the coarsening. The rate of these growths increases with the decrease of impurities.The results mentioned above will be discussed theoretically in the 3rd report.